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The oceans are getting warmer, but tropical reef fish could acclimate

It takes just two generations to adapt to warmer ocean temperatures. Jennifer Donelson

The average temperature of the oceans has already increased significantly due to global warming and will continue to warm rapidly in coming decades. If we are going to effectively manage and conserve fish populations, we need to understand if (and how) these species will adjust to higher temperatures.

Unfortunately, our current understanding of how species might acclimate and adjust to rising temperatures is incomplete. A recent study completed by my colleagues and I will hopefully develop our understanding in this area.

Tropical species are expected to be among the groups most sensitive to environmental warming because they have evolved in a relatively stable thermal environment. In addition, “ectothermic” (cold-blooded) organisms – such as fish and lizards – are likely to be strongly impacted by rises in temperature since their body temperature varies directly with the environmental temperature.

Generally, we expect some capacity for acclimation and adaptation since we already know this occurs between populations. Differences in performance (the ability to function within the environment) can be observed between the same species living at different latitudes and therefore different temperature regimes.

But there is limited knowledge of whether species’ evolutionary response to climate change will occur quickly enough to keep up with climate change. One way to approach this question is by running long-term experiments that simulate future conditions (as we did).

Jennifer Donelson

For fish and other aquatic animals, the ability to perform aerobic activity at various temperatures relates to their growth and abundance. When aerobic capacity drops too much, the animal can no longer effectively perform activities such as feeding and avoiding predators.

Myself and my colleagues investigated this phenomenon in current-day populations of one particular coral reef fish species (Acanthochromis polyacanthus). We found that water temperature increases of +1.5°C and +3.0°C caused large reductions in aerobic capacity, which would affect the fish’s ability to persist in a warmer, future environment. We also found that when fish were grown for two generations at 1.5°C and 3.0°C greater than current-day averages, aerobic capacity no longer declined and was equal to that of the current-day population.

In other words, we found that over two generations, tropical reef fish can acclimate to the warmer water temperatures we could see as a result of global warming.

But there are likely to be penalties for fish that successfully adjust to higher temperatures. Our observations suggest the acclimatised offspring are smaller on average than their parents and might not be able to reproduce at the same rate as their predecessors. We also don’t know whether this rapid ability to cope with increased temperatures is common among coral reef fish species.

Our study reveals that transgenerational acclimation is a potentially important mechanism for coping with rapid climate change. Such acclimation may allow some fish populations to persist across their current range, instead of having to move in search of cooler waters.

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